Hi, I have had my doberman for 7 years now (since he was a puppy). He was always well behaved and a sweetheart. I never worried about him being around anybody, except kids. He never had an opportunity to be around kids as a puppy or a young adult, so I was always leary about him being around kids. He would protect me by barking at strangers, unless I told him it was ok, that the "new" person was a friend, or he just sensed it by my behavior. He never bit anyone or showed aggression. Like I said, I never worried about him being around others. Two years ago I adopted a rescue and he started acting a bit different. I thought it was just him adjusting. It has been 2 years and his behavior has just gotten worse. He has now "nipped"/"bitten" 3 people. All 3 were people that are friends of mine and came either into my house or stood outside in the driveway with us. He acted fine upon meeting them and none of them were afraid of him and they were petting him and he was being friendly and all of the sudden, it was like a firecracker went off and scared him, he just became aggressive. I don't know what is going on and I don't know what to do about it. Can anyone help me by providing their thoughts or advice?
Thank you so much!
Jeanine in SC

The very first thing I would do is have a full workup by your vet - I would want a full bloodwork done and a full thyroid panel - not just a T4. Low thyroid is so common in this breed, and it can have an effect on their temperament.

Changes in behavior should always trigger a veterinary check-up. As MaryJo said, you need a full work up. Health can affect behavior. It could be something like thyroid, although the effect of that on behavior is often overstated. It could also be that he now has pain with something like arthritis, and his tolerance for small triggers is now less than it used to be, and while he used to be able to inhibit his behavior, he no longer does. I would recommend working with a veterinarian that has a good understanding of behavior.

However, since you said that he's displayed this type of behavior since you've had him, though much more mild, it could also be that it's just slowly escalated over time and you've not seen subtle signs. If you have a good veterinarian, they should be able to work with you. You need to rule out health issues (as above - is something playing into his decreased inhibition?). Many veterinarians that are skilled in behavior can recommend a good local trainer, and/or prescribe medications that can help. Training/medication can be a good option.

On top of that, safety first. He needs to be crated or contained in situations where he might bite someone. If you have company, he cannot be out with them, or, you need to muzzle train him. Good dogs wear muzzles, too. A nice, comfortable basket muzzle will prevent a bite, and keep your stress level low. See the Muzzle Up Project for how to train your dog to wear a muzzle happily: https://muzzleupproject.com/

Thank you both so much for the advice! I will definitely take him to the vet to get his blood work checked. And thank you for the advice about the muzzle. I always thought that muzzles were for bad dogs and weren't nice for the dog. But you are absolutely right. The next time, he could actually break the skin on someone and I certainly don't want that happening. I had already decided that the last time he showed aggression was the last time I could trust him to be around anyone he wasn't already completely familiar with, but I don't like the thought of locking him in a room and not allowing him to be around us (he hates to be alone). So I think the muzzle is a good idea and I will certainly educate myself on training him for that. Thanks again!

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